Light-emitting element, light emitting device, electronic appliance, and method of manufacturing the same

ABSTRACT

A light-emitting element is provided which has a light-emitting layer between a first electrode and a second electrode, where the light-emitting layer has a first layer and a second layer; the first layer contains a first organic compound and a third organic compound; the second layer contains a second organic compound and the third organic compound; the first layer is provided to be in contact with the second layer on the first electrode side; the first organic compound is an organic compound with an electron transporting property; the second organic compound is an organic compound with a hole transporting property; the third organic compound has an electron trapping property; and light emission from the third organic compound can be obtained when voltage is applied to the first electrode and the second electrode so that the potential of the first electrode is higher than that of the second electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current excitation light-emittingelement, a light-emitting device, an electronic appliance each havingthe light-emitting element, and method of manufacturing the same.

2. Description of the Related Art

In recent years, a light-emitting element utilizing electroluminescencehas been actively researched and developed. In a basic structure of sucha light-emitting element, a substance with a light-emitting property issandwiched between a pair of electrodes. When voltage is applied to thiselement, light emission can be obtained from the substance with alight-emitting property.

Since such a light-emitting element is a self-luminous type, there areadvantages in that it has higher visibility of pixels than liquidcrystal displays, there is no need to use a backlight, and the like.Thus, such a light-emitting element is considered to be suitable as aflat panel display element. In addition, such a light-emitting elementcan be manufactured to be thin and light-weight, which is also a greatadvantage. Moreover, very high response speed is also one of features ofsuch a light-emitting element.

Furthermore, since such a light-emitting element can be formed in a filmform, planar light emission can be easily obtained when a large-areaelement is formed. This cannot be easily achieved with a point lightsource typified by an incandescent lamp or an LED, or a linear lightsource typified by a fluorescent lamp, and thus, such a light-emittingelement has high utility value as a plane light source which isapplicable to a lighting system or the like.

Light-emitting elements utilizing electroluminescence are broadlyclassified according to whether they use an organic compound or aninorganic compound as a substance with a light-emitting property.

When an organic compound is used for a substance with a light-emittingproperty, electrons and holes are injected into a layer containing anorganic compound with a light-emitting property from a pair ofelectrodes by voltage application to a light-emitting element, so thatcurrent flows therethrough. Then, when carriers (electrons and holes)are recombined, the organic compound with a light-emitting property isin an excited state, and when the excited state returns to a groundstate, light is emitted. Because of such a mechanism, such alight-emitting element is referred to as a current excitationlight-emitting element.

Note that there are a singlet excited state and a triplet excited stateas the types of the excited states obtained by an organic compound.Light emission from the singlet excited state is referred to asfluorescence, and light emission from the triplet excited state isreferred to as phosphorescence.

In improving element characteristics of such a light-emitting element,there are a lot of problems depending on a material, and in order tosolve the problems, improvement of an element structure, development ofa material, and the like have been carried out.

For example, in Non-Patent Document 1 (Tetsuo Tsutsui and eight others,Japanese Journal of Applied Physics, vol. 38, L1502-1504 (1999)), a holeblocking layer is provided, so that a light-emitting element using aphosphorescent material efficiently emits light.

SUMMARY OF THE INVENTION

However, as described in Non-Patent Document 1, a hole blocking layerdoes not have durability, and a light-emitting element has a very shortlifetime. Thus, development of a light-emitting element with high lightemission efficiency and a long lifetime has been desired. In view of theforegoing, it is an object of the present invention to provide alight-emitting element having high light emission efficiency. It is anobject to provide a light-emitting element having a long lifetime; alight-emitting element having high light emission efficiency and a longlifetime; a light-emitting device and an electronic appliance eachhaving high light emission efficiency; and a light-emitting device andan electronic appliance each having a long lifetime.

One feature of the present invention is a light-emitting element whichincludes a light-emitting layer between a first electrode and a secondelectrode, where the light-emitting layer includes a first layer and asecond layer; the first layer contains a first organic compound and athird organic compound; the second layer contains a second organiccompound and the third organic compound; the first layer is provided tobe in contact with the second layer on the first electrode side; thefirst organic compound is an organic compound with an electrontransporting property; the second organic compound is an organiccompound with a hole transporting property; the third organic compoundhas an electron trapping property; and light emission from the thirdorganic compound can be obtained when voltage is applied to the firstelectrode and the second electrode so that the potential of the firstelectrode is higher than that of the second electrode.

Another feature of the present invention is a light-emitting elementwhich includes a light-emitting layer between a first electrode and asecond electrode, where the light-emitting layer includes a first layerand a second layer; the first layer contains a first organic compoundand a third organic compound; the second layer contains a second organiccompound and the third organic compound; the first layer is provided tobe in contact with the second layer on the first electrode side; thefirst organic compound is an organic compound with an electrontransporting property; the second organic compound is an organiccompound with a hole transporting property; the lowest unoccupiedmolecular orbital level (LUMO level) of the third organic compound islower than that of the second organic compound by greater than or equalto 0.3 eV; and light emission from the third organic compound can beobtained when voltage is applied to the first electrode and the secondelectrode so that the potential of the first electrode is higher thanthat of the second electrode.

Another feature of the present invention is a light-emitting elementwhich includes an electron transporting layer and a hole transportinglayer between a first electrode and a second electrode; and a firstlayer and a second layer between the electron transporting layer and thehole transporting layer, where the first layer contains a first organiccompound and a third organic compound; the second layer contains asecond organic compound and the third organic compound; the first layeris provided to be in contact with the second layer on the firstelectrode side; the first organic compound is an organic compound withan electron transporting property; the second organic compound is anorganic compound with a hole transporting property; the third organiccompound has an electron trapping property; and light emission from thethird organic compound can be obtained when voltage is applied to thefirst electrode and the second electrode so that the potential of thefirst electrode is higher than that of the second electrode.

Another feature of the present invention is a light-emitting elementwhich includes an electron transporting layer and a hole transportinglayer between a first electrode and a second electrode; and a firstlayer and a second layer between the electron transporting layer and thehole transporting layer, where the first layer contains a first organiccompound and a third organic compound; the second layer contains asecond organic compound and the third organic compound; the first layeris provided to be in contact with the second layer on the firstelectrode side; the first organic compound is an organic compound withan electron transporting property; the second organic compound is anorganic compound with a hole transporting property; the lowestunoccupied molecular orbital level (LUMO level) of the third organiccompound is lower than that of the second organic compound by greaterthan or equal to 0.3 eV; and light emission from the third organiccompound can be obtained when voltage is applied to the first electrodeand the second electrode so that the potential of the first electrode ishigher than that of the second electrode.

In the above structure, the second organic compound is preferably anarylamine derivative or a carbazole derivative.

Another feature of the present invention is a light-emitting elementwhich includes a light-emitting layer between a first electrode and asecond electrode, where the light-emitting layer includes a first layerand a second layer; the first layer contains a first organic compoundand a third organic compound; the second layer contains a second organiccompound and the third organic compound; the first layer is provided tobe in contact with the second layer on the first electrode side; thefirst organic compound is an organic compound with an electrontransporting property; the second organic compound is an organiccompound with a bipolar property; the third organic compound has anelectron trapping property; and light emission from the third organiccompound can be obtained when voltage is applied to the first electrodeand the second electrode so that the potential of the first electrode ishigher than that of the second electrode.

Another feature of the present invention is a light-emitting elementwhich includes a light-emitting layer between a first electrode and asecond electrode, where the light-emitting layer includes a first layerand a second layer; the first layer contains a first organic compoundand a third organic compound; the second layer contains a second organiccompound and the third organic compound; the first layer is provided tobe in contact with the second layer on the first electrode side; thefirst organic compound is an organic compound with an electrontransporting property; the second organic compound is an organiccompound with a bipolar property; the lowest unoccupied molecularorbital level (LUMO level) of the third organic compound is lower thanthat of the second organic compound by greater than or equal to 0.3 eV;light emission from the third organic compound can be obtained whenvoltage is applied to the first electrode and the second electrode sothat and the potential of the first electrode is higher than that of thesecond electrode.

Another feature of the present invention is a light-emitting elementwhich includes an electron transporting layer and a hole transportinglayer between a first electrode and a second electrode; and a firstlayer and a second layer between the electron transporting layer and thehole transporting layer, where the first layer contains a first organiccompound and a third organic compound; the second layer contains asecond organic compound and the third organic compound; the first layeris provided to be in contact with the second layer on the firstelectrode side; the first organic compound is an organic compound withan electron transporting property; the second organic compound is anorganic compound with a bipolar property; the third organic compound hasan electron trapping property; and light emission from the third organiccompound can be obtained when voltage is applied to the first electrodeand the second electrode so that the potential of the first electrode ishigher than that of the second electrode.

Another feature of the present invention is a light-emitting elementwhich includes an electron transporting layer and a hole transportinglayer between a first electrode and a second electrode; and a firstlayer and a second layer between the electron transporting layer and thehole transporting layer, where the first layer contains a first organiccompound and a third organic compound; the second layer contains asecond organic compound and the third organic compound; the first layeris provided to be in contact with the second layer on the firstelectrode side; the first organic compound is an organic compound withan electron transporting property; the second organic compound is anorganic compound with a bipolar property; the lowest unoccupiedmolecular orbital level (LUMO level) of the third organic compound islower than that of the second organic compound by greater than or equalto 0.3 eV; and light emission from the third organic compound can beobtained when voltage is applied to the first electrode and the secondelectrode so that the potential of the first electrode is higher thanthat of the second electrode.

In the above structure, the second organic compound preferably has anarylamine skeleton and a quinoxaline skeleton.

In the above structure, the thickness of the first layer is preferablythe same as that of the second layer, or thinner than that of the secondlayer.

In the above structure, the third organic compound preferably has anelectron-withdrawing group.

In the above structure, the third organic compound is preferably asubstance which emits phosphorescence.

In the above structure, the third organic compound is preferably a metalcomplex which has a pyrazine skeleton or a quinoxaline skeleton and hasa metal atom of Group 9 or Group 10 of the periodic table. Inparticular, the metal atom is preferably iridium (Ir) or platinum (Pt)because iridium (Ir) or platinum (Pt) can efficiently emitphosphorescence by a heavy atom effect.

In the above structure, the third organic compound preferably has astructure represented by a general formula (1).

(In the formula, Ar represents an aryl group, each of R¹ to R³represents hydrogen, an alkyl group, or an aryl group, and R² and R³ maybe bonded to each other to form a ring. M represents either an elementbelonging to Group 9 or an element belonging to Group 10.)

In the above structure, the third organic compound preferably has astructure represented by a general formula (2).

(In the formula, each of R¹ to R³ represents hydrogen, an alkyl group,or a phenyl group, and R² and R³ may be bonded to each other to form aring. Each of R⁴ to R⁷ represents hydrogen, a halogen group, an alkylgroup, or a haloalkyl group. M represents either an element belonging toGroup 9 or an element belonging to Group 10.)

In the above structure, the third organic compound is preferably anorganic compound represented by a general formula (3).

(In the formula, Ar represents an aryl group, each of R¹ to R³represents hydrogen, an alkyl group, or an aryl group, and R² and R³ maybe bonded to each other to form a ring. M is a central metal andrepresents either an element belonging to Group 9 or an elementbelonging to Group 10. L represents a monoanionic ligand. When thecentral metal is an element belonging to Group 9, n is 2, and when thecentral metal is an element belonging to Group 10, n is 1.)

In the above structure, the third organic compound is preferably anorganic compound represented by a general formula (4).

(In the formula, each of R¹ to R³ represents hydrogen, an alkyl group,or a phenyl group, and R² and R³ may be bonded to each other to form aring. Each of R⁴ to R⁷ represents hydrogen, a halogen group, an alkylgroup, or a haloalkyl group. M is a central metal and represents eitheran element belonging to Group 9 or an element belonging to Group 10. Lrepresents a monoanionic ligand. When the central metal is an elementbelonging to Group 9, n is 2, and when the central metal is an elementbelonging to Group 10, n is 1.).

In the above structure, the monoanionic ligand is preferably any of amonoanionic bidentate chelate ligand having a beta-diketone structure, amonoanionic bidentate chelate ligand having a carboxyl group, amonoanionic bidentate chelate ligand having a phenolic hydroxyl group,and a monoanionic bidentate chelate ligand in which two ligand elementsare both nitrogen.

In the above structure, the monoanionic ligand is preferably monoanionicligands represented by the following structural formulas (L1) to (L8).

In the above structure, M is preferably iridium (Ir) or platinum (Pt)because iridium (Ir) or platinum (Pt) can efficiently emitphosphorescence by a heavy atom effect.

In the above structure, the ionization potential of the first organiccompound is preferably less than or equal to 6.0 eV.

In the above structure, the first organic compound is preferably a metalcomplex.

In the above structure, the thickness of the first layer is preferablygreater than or equal to 5 nm and less than or equal to 30 nm.

In the above structure, the thickness of the second layer is preferablygreater than or equal to 5 nm and less than or equal to 30 nm.

The present invention includes a light-emitting device having theabove-described light-emitting element in its category. Thelight-emitting device in this specification refers to an image displaydevice, a light-emitting device, or a light source (also refers to alighting system). In addition, the light-emitting element also refers tothe following: a module in which a connector, for example, an FPC(flexible printed circuit), a TAB (tape automated bonding) tape, or aTCP (tape carrier package), is attached to a panel including alight-emitting element; a module in which a printed circuit board isprovided at an end of a TAB tape or a TCP; and a module in which an IC(integrated circuit) is directly mounted on a light-emitting element bya COG (chip on glass) method.

The present invention also includes an electronic appliance in which thelight-emitting element of the present invention is used for a displayportion in its category. Thus, an electronic appliance of the presentinvention has a feature of having a display portion which is providedwith the above-described light-emitting element and a control unit forcontrolling light emission of the light-emitting element.

The light-emitting element of the present invention has good carrierbalance and high recombination probability of carriers. Thus, alight-emitting element with high light emission efficiency can beobtained. In addition, since the light-emitting element has good carrierbalance, a light-emitting element with a long lifetime can be obtained.Moreover, a light-emitting element with high light emission efficiencyand a long lifetime can be obtained.

When the light-emitting element of the present invention is applied to alight-emitting device and an electronic appliance, a light-emittingdevice and an electronic appliance each having high light emissionefficiency can be obtained. In addition, a light-emitting device and anelectronic appliance each having a long lifetime can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are diagrams explaining a light-emitting element of thepresent invention;

FIG. 2 is a diagram explaining a light-emitting element of the presentinvention;

FIG. 3 is a diagram explaining a light-emitting element of the presentinvention;

FIG. 4 is a diagram explaining a light-emitting element of the presentinvention;

FIGS. 5A and 5B are diagrams explaining a light-emitting device of thepresent invention;

FIGS. 6A and 6B are diagrams explaining a light-emitting device of thepresent invention;

FIGS. 7A to 7D are diagrams explaining electronic appliances of thepresent invention;

FIG. 8 is a diagram explaining an electronic appliance of the presentinvention;

FIG. 9 is a diagram explaining an electronic appliance of the presentinvention;

FIG. 10 is a diagram explaining an electronic appliance of the presentinvention;

FIG. 11 is a diagram explaining a lighting system of the presentinvention;

FIG. 12 is a diagram explaining a lighting system of the presentinvention;

FIG. 13 is a diagram explaining a light-emitting element of Embodiments;

FIG. 14 is a graph showing current density-luminance characteristics ofa light-emitting element manufactured in Embodiment 1;

FIG. 15 is a graph showing voltage-luminance characteristics of alight-emitting element manufactured in Embodiment 1;

FIG. 16 is a graph showing luminance-current efficiency characteristicsof a light-emitting element manufactured in Embodiment 1;

FIG. 17 is a graph showing an emission spectrum of a light-emittingelement manufactured in Embodiment 1;

FIG. 18 is a graph showing current density-luminance characteristics ofa light-emitting element manufactured in Embodiment 2;

FIG. 19 is a graph showing voltage-luminance characteristics of alight-emitting element manufactured in Embodiment 2;

FIG. 20 is a graph showing luminance-current efficiency characteristicsof a light-emitting element manufactured in Embodiment 2;

FIG. 21 is a graph showing an emission spectrum of a light-emittingelement manufactured in Embodiment 2;

FIG. 22 is a graph showing time dependence of normalized luminance of alight-emitting element manufactured in Embodiment 2;

FIG. 23 is a graph showing current density-luminance characteristics ofa light-emitting element manufactured in Embodiment 3;

FIG. 24 is a graph showing voltage-luminance characteristics of alight-emitting element manufactured in Embodiment 3;

FIG. 25 is a graph showing luminance-current efficiency characteristicsof a light-emitting element manufactured in Embodiment 3;

FIG. 26 is a graph showing an emission spectrum of a light-emittingelement manufactured in Embodiment 3;

FIG. 27 is a diagram explaining a conventional light-emitting element;

FIG. 28 is a graph showing reduction reaction characteristics of BPAPQ;

FIG. 29 is a graph showing reduction reaction characteristics ofIr(Fdpq)₂(acac); and

FIG. 30 is a graph showing reduction reaction characteristics ofIr(tppr)₂(acac).

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode

Embodiment Modes of the present invention will be hereinafter explainedin detail with reference to the drawings. However, the present inventionis not limited to the following explanation, and it is easily understoodby those skilled in the art that modes and details of the presentinvention can be modified in various ways without departing from thespirit and scope of the present invention. Therefore, the presentinvention should not be interpreted as being limited to the followingdescription of Embodiment Modes.

Note that in this specification, the term “composite” means a state inwhich charges can be transferred between materials by not only simplemixture of two materials but also by mixture of a plurality ofmaterials.

Embodiment Mode 1

One mode of a light-emitting element of the present invention will beexplained with reference to FIG. 1A.

The light-emitting element of the present invention includes a pluralityof layers between a pair of electrodes. The plurality of layers isstacked in combination of a layer formed of a substance with a highcarrier injecting property and a layer formed of a substance with a highcarrier transporting property so that a light-emitting region is formedin a portion away from the electrodes, in other words, so that carriersare recombined in a portion away from the electrodes.

In this embodiment mode, a light-emitting element includes a firstelectrode 102, a second electrode 104, and an EL layer 103 providedbetween the first electrode 102 and the second electrode 104. Note thatin this embodiment mode, explanation is made on the assumption that thefirst electrode 102 functions as an anode and the second electrode 104functions as a cathode. That is, the explanation is made on theassumption that light emission can be obtained when voltage is appliedto the first electrode 102 and the second electrode 104 so that thepotential of the first electrode 102 is higher than that of the secondelectrode 104.

The substrate 101 is used as a support of the light-emitting element.For example, glass, plastic, or the like can be used for the substrate101. Note that other materials may also be used as long as they functionas a support in a manufacturing process of the light-emitting element.

The first electrode 102 is preferably formed of a metal, an alloy, aconductive compound, a mixture thereof, or the like having a high workfunction (specifically, a work function of greater than or equal to 4.0eV). Specifically, for example, indium tin oxide (ITO), indium tin oxidecontaining silicon or silicon oxide, indium zinc oxide (IZO), indiumoxide containing tungsten oxide and zinc oxide (IWZO), and the like aregiven. Although such conductive metal oxide films are generally formedby sputtering, they may also be formed by a sol-gel method. For example,indium zinc oxide (IZO) can be formed by sputtering using a target inwhich 1 to 20 wt % of zinc oxide is added to indium oxide. In addition,indium oxide containing tungsten oxide and zinc oxide (IWZO) can beformed by sputtering using a target in which 0.5 to 5 wt % of tungstenoxide and 0.1 to 1 wt % of zinc oxide are added to indium oxide. Otherthan these, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd), a nitride of a metal material (e.g., titanium nitride),and the like are given.

The EL layer 103 described in this embodiment mode includes a holeinjecting layer 111, a hole transporting layer 112, a light-emittinglayer 113, an electron transporting layer 114, and an electron injectinglayer 115. Note that the EL layer 103 is acceptable as long as it hasthe light-emitting layer described in this embodiment mode, and astacked structure of layers other than the light-emitting layer 113 isnot particularly limited. That is, there is no particular limitation ona stacked structure of the EL layer 103. The EL layer 103 may be formedin an appropriate combination of the light-emitting layer described inthis embodiment mode and layers containing a substance with a highelectron transporting property, a substance with a high holetransporting property, a substance with a high electron injectingproperty, a substance with a high hole injecting property, a substancewith a bipolar property (a substance with high electron and holetransporting properties), and the like. For example, the EL layer 103can be formed in an appropriate combination of a hole injecting layer, ahole transporting layer, a light-emitting layer, an electrontransporting layer, an electron injecting layer, and the like. Amaterial forming each layer is specifically described below.

The hole injecting layer 111 is a layer containing a substance with ahigh hole injecting property. As the substance with a high holeinjecting property, the following can be used: molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, andthe like. The hole injecting layer 111 can also be formed of aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) and copper phthalocyanine (CuPc), a polymer such aspoly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS), orthe like.

Alternatively, the hole injecting layer 111 can be formed using acomposite material in which a substance with an acceptor property ismixed into a substance with a high hole transporting property. Note thatwhen the composite material in which a substance with an acceptorproperty is mixed into a substance with a high hole transportingproperty is used, a material for forming an electrode can be selectedregardless of work function of the electrode. That is, besides amaterial with a high work function, a material with a low work functionmay also be used for the first electrode 102. As the substance with anacceptor property, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ), chloranil, and the like can be given. Inaddition, transition metal oxide can be given. Moreover, an oxide of ametal belonging to Groups 4 to 8 of the periodic table can also begiven. In particular, it is preferable to use vanadium oxide, niobiumoxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, and rhenium oxide because they have high electronaccepting properties. Above all, molybdenum oxide is particularlypreferable because it is stable even in the atmospheric air, has a lowhygroscopic property, and is easy to be handled.

As an organic compound used for the composite material, variouscompounds such as aromatic amine compounds, carbazole derivatives,aromatic hydrocarbons, and high molecular compounds (e.g., oligomer,dendrimer, or polymer) can be used. The organic compound used for thecomposite material is preferably an organic compound with a high holetransporting property. Specifically, a compound with a hole mobility ofgreater than or equal to 10⁻⁶ cm²/Vs is preferably used. However, othercompounds may also be used as long as the hole transporting propertiesthereof are higher than the electron transporting properties thereof.Specific organic compounds that can be used for the composite materialare given below.

As the aromatic amine compound, the following can be given, for example:N,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA); 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB);4,4′-bis(N-{4-[N-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD);1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B); and the like.

As the carbazole derivative that can be used for the composite material,the following can be specifically given:3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2);3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); and the like.

In addition, the following can be used: 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP); 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB); 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA);1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene; and the likecan be used.

As the aromatic hydrocarbon that can be used for the composite material,the following can be given, for example:2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA);2-tert-butyl-9,10-di(1-naphthyl)anthracene;9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA);2-tert-butyl-9,10-dis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA);9,10-di(2-naphthyl)anthracene (abbreviation: DNA);9,10-diphenylanthracene (abbreviation: DPAnth); 2-tert-butylanthracene(abbreviation: t-BuAnth); 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA);9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene;9,10-bis[2-(1-naphthyl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene;2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene; 9,9′-bianthryl;10,10′-diphenyl-9,9′-bianthryl;10,10′-bis(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; anthracene;tetracene; rubrene; perylene; 2,5,8,11-tetra(tert-butyl)perylene; andthe like. In addition to these, pentacene, coronene, and the like canalso be used. As described above, it is more preferable to use aromatichydrocarbon which has a hole mobility of greater than or equal to 1×10⁻⁶cm²/Vs and has 14 to 42 carbon atoms.

The aromatic hydrocarbon that can be used for the composite material mayhave a vinyl skeleton. As the aromatic hydrocarbon having a vinyl group,the following are given for example: 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi); 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA); and the like.

In addition, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyl triphenylamine) (abbreviation:PVTPA) can also be used.

The hole transporting layer 112 is a layer containing a substance with ahigh hole transporting property. As the substance with a high holetransporting property, aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-dipheny-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), andN,N′-bis(spiro-9,9′-bifluorene-2-yl)-N,N′-diphenylbenzidine(abbreviation: BSPB) can be used. The substances described here aremainly substances which have hole mobility of greater than or equal to10⁻⁶ cm²/Vs. However, other substances may also be used as long as thehole transporting properties thereof are higher than the electrontransporting properties thereof. Note that the layer containing asubstance with a high hole transporting property is not limited to asingle layer but may have a stacked structure of two or more layersformed of the above-described substances.

The light-emitting layer 113 is a layer containing a substance with ahigh light-emitting property. In the light-emitting element of thepresent invention, the light-emitting layer includes a first layer 121and a second layer 122. The first layer 121 contains a first organiccompound and a third organic compound. The second layer 122 contains asecond organic compound and the third organic compound. The first layer121 is provided to be in contact with the second layer 122 on the firstelectrode side, that is, on the anode side.

The first organic compound contained in the first layer 121 is asubstance with an electron transporting property, in which an electrontransporting property is higher than a hole transporting property. Asthe first organic compound, a metal complex having a quinoline skeletonor a benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(abbreviation: BeBq₂); orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq); or the like can be used. In addition to these, a metal complexhaving an oxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolate]zinc (abbreviation: Zn(BTZ)₂);or the like can be also used. Furthermore, besides the metal complex,the following can be used:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD); 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7);3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ); bathophenanthroline (abbreviation: BPhen);bathocuproin (abbreviation: BCP); and the like.

The first organic compound is preferably a substance into which holesare injected. That is, the ionization potential of the first organiccompound is preferably less than or equal to 6.0 eV. If the firstorganic compound is a compound into which holes are injected, drivingvoltage is prevented from being very high, so that the effect of thepresent invention can be obtained. Of the above-described materials, themetal complexes such as Alq (ionization potential: 5.65 eV), BAlq(ionization potential: 5.70 eV), Zn(BOX)₂ (ionization potential: 5.62eV), and Zn(BTZ)₂ (ionization potential: 5.49 eV) have ionizationpotential of less than or equal to 6.0 eV and holes are comparativelyeasily injected although they have electron transporting properties;thus, these metal complexes are preferable as the first organiccompound.

The second organic compound contained in the second layer 122 is asubstance with a hole transporting property. Specifically, the secondorganic compound is a substance in which a hole transporting property ishigher than an electron transporting property, a so-called substancewith a hole transporting property, or a substance with a bipolarproperty which has both an electron transporting property and a holetransporting property. That is, it is acceptable as long as the secondorganic compound is a substance with a hole transporting property.

As the substance with a hole transporting property, arylaminederivatives or carbazole derivatives can be given. Specifically, as thearylamine derivatives, the following can be given:4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),N,N-bis(spiro-9,9′-bifluorene-2-yl)-N,N′-diphenylbenzidine(abbreviation: BSPB), and the like. In addition, the following can begiven:N,N′-bis(4-methylphenyl)(p-tolyl)-N,N′-diphenyl-p-phenylenediamine(abbreviation: DTDPPA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like. As the carbazole derivatives, thefollowing can be given: 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

As the substance with a bipolar property, a substance having anarylamine skeleton and a quinoxaline skeleton in the same molecule canbe given. Specifically, the following can be given:2,3-bis(4-diphenylaminophenyl)quinoxaline (abbreviation: TPAQn),2,3-bis{4-[N-(4-biphenyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ),2,3-bis{4-[N,N-di(4-biphenylyl)amino]phenyl}quinoxaline (abbreviation:BBAPQ),4,4′-(quinoxaline-2,3-diyl)bis{N-[4-(9-carbazolyl)phenyl]-N-phenylbenzeneamine}(abbreviation:YGAPQ),N,N′-(quinoxaline-2,3-diyldi-4,1-phenylene)bis(N-phenyl-9-phenylcarbazole-3-amine)(abbreviation: PCAPQ), and the like.

The second organic compound preferably has a certain amount of electrontransporting property because the third organic compound with anelectron trapping property is added thereto. Therefore, the secondorganic compound is preferably an organic compound with a bipolarproperty.

In the case where a substance with a bipolar property is used as thesecond organic compound, the first layer 121 is preferably as thick asthe second layer 122, or thinner than the second layer 122. A substancewith a bipolar property is contained in the second layer 122, so thatincrease in driving voltage can be suppressed even if the second layer122 is made thicker than the first layer 121.

The third organic compound contained in the first layer 121 and thesecond layer 122 is a substance with a high light-emitting property.Various materials can be used for the third organic compound. Inparticular, the third organic compound is preferably a substance with anelectron trapping property. When the third organic compound is asubstance with a high electron trapping property, more profound effectcan be obtained by application of the present invention, which ispreferable. Thus, the lowest unoccupied molecular orbital level (LUMOlevel) of the third organic compound is preferably lower than that ofthe second organic compound by greater than or equal to 0.3 eV. Asubstance with an electron-withdrawing group is preferable because thelowest unoccupied molecular orbital level (LUMO level) thereof tends tobe lowered. As the electron-withdrawing group, a halogen group such as afluoro group; a cyano group; a haloalkyl group such as a trifluoromethylgroup; a carbonyl group; and the like are given.

Specifically, as a substance which emits light (fluorescence) from asinglet excited state, the following can be given: a substance whichexhibits light emission of blue to blue green, such as acridone,coumarin 102, coumarin 6H, coumarin 480D, or coumarin 30; a substancewhich exhibits light emission of blue green to yellow green, such asN,N′-dimethylquinacridone (abbreviation: DMQd),N,N′-diphenylquinacridone (abbreviation: DPQd),9,18-dihydro-benzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-1),9,18-dimethyl-9,18-dihydro-benzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-2), coumarin 30, coumarin 6, coumarin 545T, orcoumarin 153; a substance which exhibits light emission of yellow greento yellow orange, such as DMQd or(2-{2-[4-(9H-carbazol-9-yl)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCMCz); a substance which exhibits light emission oforange to red, such as(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),{2-(1,1-dimethylethyl)-6-[2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), or Nile red; or the like. As a substance whichemits light (phosphorescence) from a triplet excited state, thefollowing can be given: bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate(abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: FIrpic),bis[2-(3′,5′-bistrifluoromethyl-phenyl)-pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: Ir(CF₃ ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac), or the like. In particular, asubstance which emits phosphorescence from a triplet excited state ispreferable because it has high light emission efficiency.

In addition, it is preferable that the third organic compound be a metalcomplex which has a pyrazine skeleton or a quinoxaline skeleton and havea metal atom of Group 9 or Group 10 of the periodic table. The metalcomplex having such a structure is preferable because it emitsphosphorescence from a triplet excited state. In particular, the metalatom is preferably iridium (Ir) or platinum (Pt) because iridium (Ir) orplatinum (Pt) can efficiently emit phosphorescence by a heavy atomeffect.

Specifically, an organic compound having a structure represented by ageneral formula (1) can be given as the third organic compound.

(In the formula, Ar represents an aryl group, each of R¹ to R³represents hydrogen, an alkyl group, or an aryl group, and R² and R³ maybe bonded to each other to form a ring. The ring may be an alicyclicring or a condensed ring. M represents either an element belonging togroup 9 or an element belonging to Group 10.)

In addition, an organic compound having a structure represented by ageneral formula (2) can be given as the third organic compound.

(In the formula, each of R¹ to R³ represents hydrogen, an alkyl group,or a phenyl group, and R² and R³ may be bonded to each other to form aring. The ring may be an alicyclic ring or a condensed ring. Each of R⁴to R⁷ represents hydrogen, a halogen group, an alkyl group, or ahaloalkyl group. M represents either an element belonging to Group 9 oran element belonging to Group 10.)

More specifically, the third organic compound is preferably an organiccompound represented by a general formula (3).

(In the formula, Ar represents an aryl group, each of R¹ to R³represents hydrogen, an alkyl group, or an aryl group, and R² and R³ maybe bonded to each other to form a ring. The ring may be an alicyclicring or a condensed ring. M is a central metal and represents either anelement belonging to Group 9 or an element belonging to Group 10. Lrepresents a monoanionic ligand. When the central metal is an elementbelonging to Group 9, n is 2, and when the central metal is an elementbelonging to Group 10, n is 1.)

In addition, the third organic compound is preferably an organiccompound represented by a general formula (4).

(In the formula, each of R¹ to R³ represents hydrogen, an alkyl group,or a phenyl group, and R² and R³ may be bonded to each other to form aring. The ring may be an alicyclic ring or a condensed ring. Each of R⁴to R⁷ represents hydrogen, a halogen group, an alkyl group, or ahaloalkyl group. M is a central metal and represents either an elementbelonging to Group 9 or an element belonging to Group 10. L represents amonoanionic ligand. When the central metal is an element belonging toGroup 9, n is 2, and when the central metal is an element belonging toGroup 10, n is 1.)

Note that the monoanionic ligand L in each of the general formulas (3)and (4) is preferably either one of the following because of highcoordinating ability: a monoanionic bidentate chelate ligand having abeta-diketone structure, a monoanionic bidentate chelate ligand having acarboxyl group, a monoanionic bidentate chelate ligand having a phenolichydroxyl group, and a monoanionic bidentate chelate ligand in which twoligand elements are both nitrogen. More preferably, the monoanionicligand L is monoanionic ligands represented by the following structuralformulas (L1) to (L8). These ligands are useful because they have highcoordinating ability and are available at a low price.

For more efficient emission of phosphorescence, a heavy metal ispreferable as a central metal in terms of a heavy atom effect. Thus, thecentral metal M is preferably iridium or platinum in the above-describedorganometallic complex of the present invention. When the central metalM is iridium, the heat resistance of the organometallic complex isimproved; thus, iridium is particularly preferable for the central metalM.

Specifically, organic compounds represented by general formulas (11) to(38) are given as the organic compounds represented by the generalformulas (3) and (4).

It is preferable that the amount of the first organic compound containedin the first layer 121 be larger than that of the third organiccompound. In addition, it is preferable that the amount of the secondorganic compound contained in the second layer 122 be larger than thatof the third organic compound.

Here, FIG. 3 shows an example of a band diagram of the light-emittingelement of the present invention shown in FIGS. 1A to 1C. In FIG. 3,electrons injected from the second electrode 104 pass through theelectron injecting layer 115 and the electron transporting layer 114 tobe injected into the second layer 122. The electrons injected into thesecond layer 122 are trapped in a substance with a high electrontrapping property, so that the speed of the electrons moving through thesecond layer 122 becomes slow.

On the other hand, holes injected from the first electrode 102 passthrough the hole injecting layer 111 and the hole transporting layer 112to be injected into the first layer 121. The holes injected into thefirst layer 121 move through the layer in which a large number of firstorganic compounds with an electron transporting property are contained,so that the moving speed of the holes becomes slow.

Accordingly, carrier balance between the holes and the electrons isimproved and a recombination region 131 is formed in the vicinity of aninterface between the first layer 121 and the second layer 122. Inaddition, when the carrier balance is improved, light emissionefficiency is increased. Moreover, the life of the light-emittingelement is extended.

Since the moving speeds of the holes and electrons become slow, therecombination region 131 is enlarged. Accordingly, exciton density isdecreased and T-T annihilation is unlikely to occur even when asubstance which emits phosphorescence is used as a substance with a highlight-emitting property. In addition, since the recombination region 131is located in the vicinity of the center of the light-emitting layer,light emission from the hole transporting layer 112 or the electrontransporting layer 114 can be suppressed and light emission with goodcolor purity can be obtained. Moreover, the holes can be prevented fromreaching the electron transporting layer 114 without recombination orthe electrons can be prevented from reaching the hole transporting layer112 without recombination; thus, deterioration of the hole transportinglayer 112 due to the injection of the electrons or deterioration of theelectron transporting layer 114 due to the injection of the holes can besuppressed. That is, a light-emitting element with a long lifetime canbe obtained.

FIG. 27 shows an example of a band diagram of a conventional structurein which a first layer is not provided. Electrons injected from a secondelectrode 204 pass through an electron injecting layer 215 and anelectron transporting layer 214 to be injected into the second layer222. The electrons injected into the second layer 222 are trapped in athird organic compound with a high electron trapping property, so thatthe speed of the electrons moving through the second layer 222 becomesslow.

On the other hand, holes injected from the first electrode 201 passthrough the hole injecting layer 211 and the hole transporting layer 212to be injected into the second layer 222. The holes easily move throughthe second layer 222 because a large number of second organic compoundswith a hole transporting property are contained in the second layer 222.Accordingly, the holes reach the vicinity of an interface between theelectron transporting layer 214 and the second layer 222 and arecombination region 231 is located in the vicinity of the interfacebetween the second layer 222 and the electron transporting layer 214. Inthis case, the recombination region 231 is very narrow, so that excitondensity is increased. Thus, when a substance which emits phosphorescenceis used as a substance with a high light-emitting property, T-Tannihilation occurs and light emission efficiency is decreased. Inaddition, since the recombination region 231 is located in the vicinityof the interface with the electron transporting layer 214, lightemission from the electron transporting layer 214 could occur. When theelectron transporting layer 214 emits light, desired color cannot beobtained and light emission efficiency of a light-emitting element isdecreased. Moreover, since the holes reach the electron transportinglayer 214 without recombination, deterioration of the electrontransporting layer 214 due to the injection of the holes occurs and thelife of the light-emitting element is shortened.

When only the first layer is provided and the second layer is notprovided, driving voltage becomes very high. That is, the drivingvoltage becomes very high if an organic compound which transports holesis not contained in the light-emitting layer.

Note that the thickness of the first layer is preferably greater than orequal to 5 nm and less than or equal to 30 nm. In addition, thethickness of the second layer is preferably greater than or equal to 5nm and less than or equal to 30 nm. When the thicknesses are within thisrange, the driving voltage is not increased too much and an effect thatcarrier balance is improved can be obtained.

The electron transporting layer 114 is a layer containing a substancewith a high electron transporting property. For example, the electrontransporting layer 114 is formed of a metal complex having a quinolineskeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂); orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq); or the like can be used. In addition to these, a metal complexhaving an oxazole-based ligand or a thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolate]zinc (abbreviation: Zn(BTZ)₂); orthe like. Furthermore, besides the metal complex, the following can beused: 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD);1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7);3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ); bathophenanthroline (abbreviation: BPhen);bathocuproin (abbreviation: BCP); and the like. The substances describedhere are mainly substances which have electron mobility of greater thanor equal to 10⁻⁶ cm²/Vs. Note that substances other than the abovesubstances may be used for the electron transporting layer 114 as longas they are substances in which an electron transporting property ishigher than a hole transporting property. In addition, the electrontransporting layer 114 is not limited to a single layer but may have astacked structure of two or more layers formed of the above-describedsubstances.

In addition, the electron injecting layer 115 may be provided. For theelectron injecting layer 115, an alkali metal, an alkaline earth metal,or a compound thereof, such as lithium fluoride (LiF), cesium fluoride(CsF), or calcium fluoride (CaF₂) can be used. For example, a layerformed of a substance with an electron transporting property containingan alkali metal, an alkaline earth metal, or a compound thereof, such asa layer formed of Alq containing magnesium (Mg), can be used. Note thatwhen a layer formed of a substance with an electron transportingproperty which contains an alkali metal or an alkaline earth metal isused as the electron injecting layer 115, electrons can be efficientlyinjected from the second electrode 104, which is preferable.

For a substance which forms the second electrode 104, a metal with a lowwork function (specifically, less than or equal to 3.8 eV), an alloy, anelectrically conductive compound, a mixture thereof, or the like can beused. As a specific example of such a cathode material, the followingcan be given: an element belonging to Group 1 or Group 2 of the periodictable, that is, an alkali metal such as lithium (Li) or cesium (Cs), analkaline earth metal such as magnesium (Mg), calcium (Ca), or strontium(Sr), an alloy containing these (e.g., an MgAg alloy or an AlLi alloy),a rare-earth metal such as europium (Eu) or ytterbium (Yb), an alloycontaining these, and the like. However, a layer having a function ofpromoting electron injection is provided between the second electrode104 and the electron transporting layer 114, so that the secondelectrode 104 can be formed using various conductive materials such asAl, Ag, ITO, and indium tin oxide containing silicon or silicon oxide,regardless of their work functions.

As a formation method of the EL layer, various methods can be usedregardless of whether a dry method or a wet method. For example, avacuum evaporation method, an ink jetting method, a spin coating method,or the like may be used. A different film-formation method may be usedfor each electrode or each layer.

In the light-emitting element of the present invention having thestructure as described above, current flows by a potential differencegenerated between the first electrode 102 and the second electrode 104and holes and electrons are recombined in the EL layer 103; wherebylight is emitted. More specifically, in the light-emitting layer 113 inthe EL layer 103, a light-emitting region is formed in a region centeredaround where the first layer 121 and the second layer 122 meet.

Light emission is extracted to the outside through one or both of thefirst electrode 102 and the second electrode 104. Therefore, one or bothof the first electrode 102 and the second electrode 104 arelight-transmitting electrodes. In the case where only the firstelectrode 102 is a light-transmitting electrode, as shown in FIG. 1A,light is extracted from the substrate side through the first electrode102. In the case where only the second electrode 104 is alight-transmitting electrode, as shown in FIG. 1B, light is extractedfrom the side opposite to the substrate through the second electrode104. In the case where both the first electrode 102 and the secondelectrode 104 are light-transmitting electrodes, as shown in FIG. 1C,light is extracted from both the substrate side and the side opposite tothe substrate through the first electrode 102 and the second electrode104.

Note that a structure of the layers provided between the first electrode102 and the second electrode 104 is not limited to the above structure.A structure other than the above may be employed as long as alight-emitting region in which holes and electrons are recombined isprovided in a portion away from the first electrode 102 and the secondelectrode 104 in order to prevent quenching caused by proximity of thelight-emitting region to a metal and the light-emitting layer includesthe first layer 121 and the second layer 122.

That is, there is no particular limitation on the stacked structure ofthe layers. Layers formed of a substance with a high electrontransporting property or a high hole transporting property, a substancewith a high electron injecting property, a substance with a high holeinjecting property, a substance with a bipolar property (a substancewith a high electron transporting property and a high hole transportingproperty), a hole blocking material, and the like may be freely combinedwith the light-emitting layer of the present invention.

A light-emitting element shown in FIG. 2 has a structure in which asecond electrode 304 functioning as a cathode, an EL layer 303, and afirst electrode 302 functioning as an anode are sequentially stackedover a substrate 301. The EL layer 303 includes an electron injectinglayer 315, an electron transporting layer 314, a light-emitting layer313, a hole transporting layer 312, and a hole injecting layer 311. Thelight-emitting layer 313 includes a first layer 321 and a second layer322. The first layer 321 is provided closer to the first electrode 302functioning as an anode than the second layer 322 is.

In this embodiment mode, the light-emitting element is formed over asubstrate made of glass, plastic, or the like. A plurality of suchlight-emitting elements is manufactured over one substrate, so that apassive matrix light-emitting device can be manufactured. In addition,for example, a thin film transistor (TFT) may be manufactured over asubstrate made of glass, plastic, or the like, and a light-emittingelement which is electrically connected to the TFT may be manufactured.Accordingly, an active matrix light-emitting device in which driving ofthe light-emitting element is controlled by the TFT can be manufactured.Note that there is no particular limitation on a structure of the TFT.The TFT may be either a staggered TFT or an inversely-staggered TFT.Driver circuits formed over a TFT substrate may be formed usingn-channel and p-channel TFTs, or using either an n-channel TFT or ap-channel TFT. In addition, there is no particular limitation oncrystallinity of a semiconductor film used for the TFT. Either anamorphous semiconductor film or a crystalline semiconductor film may beused.

Since the light-emitting element of the present invention has goodcarrier balance, light emission efficiency is high. In addition, sincethe light emission efficiency is high, power consumption is low.

In addition, the light-emitting region of the light-emitting element ofthe present invention is not formed at an interface between thelight-emitting layer and the hole transporting layer or an interfacebetween the light-emitting layer and the electron transporting layer butin the vicinity of the center of the light-emitting layer; thus, thehole transporting layer or the electron transporting layer can beprevented from emitting light. Accordingly, light emission with goodcolor purity can be obtained.

The light-emitting region of the light-emitting element of the presentinvention is not formed at the interface between the light-emittinglayer and the hole transporting layer or the interface between thelight-emitting layer and the electron transporting layer but in thevicinity of the center of the light-emitting layer. Thus, thelight-emitting element is not affected by deterioration due to proximityof the light-emitting region to the hole transporting layer or theelectron transporting layer. Accordingly, a light-emitting element whichhas less deterioration and a long lifetime can be obtained.

In addition, in the light-emitting element of the present invention, therecombination region 131 is large and the exciton density is decreased;thus, T-T annihilation is unlikely to occur even when a substance whichemits phosphorescence is used as a substance with a high light-emittingproperty, and high light emission efficiency can be maintained.

Note that this embodiment mode can be appropriately combined with otherembodiment modes.

Embodiment Mode 2

In this embodiment mode, a mode of a light-emitting element of thepresent invention having a structure in which a plurality oflight-emitting units is stacked (hereinafter, referred to as a stackedelement) will be explained with reference to FIG. 4. This light-emittingelement has a plurality of light-emitting units between a firstelectrode and a second electrode.

In FIG. 4, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502. The first electrode 501 and the second electrode 502 canbe similar to those described in Embodiment Mode 1. In addition, thefirst light-emitting unit 511 and the second light-emitting unit 512 mayhave the same structure or different structures. Structures of the firstlight-emitting unit 511 and the second light-emitting unit 512 may besimilar to the structure of the EL layer described in Embodiment Mode 1.

A charge generation layer 513 contains a composite material of anorganic compound and metal oxide. This composite material of an organiccompound and metal oxide is the composite material described inEmbodiment Mode 1. The composite material contains an organic compoundand metal oxide such as V₂O₅, MoO₃, or WO₃. As the organic compound,various compounds such as aromatic amine compounds, carbazolederivatives, aromatic hydrocarbons, or high molecular compounds (e.g.,oligomer, dendrimer, or polymer) can be used. Note that as the organiccompound, an organic compound with a hole transporting property whichhas hole mobility of greater than or equal to 10⁻⁶ cm²/Vs is preferablyused. However, other substances may also be used as long as the holetransporting properties thereof are higher than the electrontransporting properties thereof. The composite material of an organiccompound and metal oxide is superior in a carrier injecting property anda carrier transporting property, and therefore, low-voltage driving andlow-current driving can be realized.

Note that the charge generation layer 513 may be formed in a combinationof a composite material of an organic compound and metal oxide withanother material. For example, the charge generation layer 513 may beformed in a combination of a layer containing the composite material ofan organic compound and metal oxide with a layer containing one compoundselected from substances with electron donating properties and acompound having a high electron transporting property. Alternatively,the charge generation layer 513 may be formed in a combination of alayer containing the composite material of an organic compound and metaloxide with a transparent conductive film.

In any case, the charge generation layer 513 sandwiched between thefirst light-emitting unit 511 and the second light-emitting unit 512 isacceptable as long as electrons are injected into a light-emitting uniton one side and holes are injected into a light-emitting unit on theother side when voltage is applied to the first electrode 501 and thesecond electrode 502.

Although this embodiment mode explains the light-emitting element havingtwo light-emitting units, the present invention can be similarly appliedto a light-emitting element in which three or more light-emitting unitsare stacked. When a plurality of light-emitting units between a pair ofelectrodes is arranged in such a manner that the plurality oflight-emitting units is partitioned with a charge generation layer likethe light-emitting element of this embodiment mode, a light-emittingelement with a long lifetime can be realized in a high luminance regionwhile maintaining low current density. In addition, when thelight-emitting element is applied to lighting, voltage drop which wouldbe caused by the resistance of an electrode material can be suppressed,and thus, homogeneous light emission in a large area can be realized.Moreover, a light-emitting device which can be driven with low voltageand consumes less electric power can be achieved.

When the light-emitting units are formed to have different emissioncolors from each other, light emission with a desired color can beobtained as a whole light-emitting element. For example, in thelight-emitting element having two light-emitting units, when an emissioncolor of the first light-emitting unit and the emission color of thesecond light-emitting unit are complementary colors, a light-emittingelement which emits white light as a whole light-emitting element can beobtained. Note that “complementary color” refers to a relationshipbetween colors which become achromatic color by being mixed. That is,white light emission can be obtained by mixture of light obtained fromsubstances which emit the light of complementary colors. The same can besaid for a light-emitting element which has three light-emitting units.For example, white light emission can be obtained as a wholelight-emitting element when the emission color of the firstlight-emitting unit is red, the emission color of the secondlight-emitting unit is green, and the emission color of the thirdlight-emitting unit is blue.

Note that this embodiment mode can be appropriately combined with otherembodiment modes.

Embodiment Mode 3

In this embodiment mode, a light-emitting device having a light-emittingelement of the present invention will be explained.

In this embodiment mode, a light-emitting device having thelight-emitting element of the present invention in a pixel portion willbe explained with reference to FIGS. 5A and 5B. FIG. 5A is a top view ofa light-emitting device, and FIG. 5B is a cross-sectional diagram takenalong lines A-A′ and B-B′ of FIG. 5A. This light-emitting deviceincludes a driver circuit portion (source side driver circuit) 601, apixel portion 602, and a driver circuit portion (gate side drivercircuit) 603 which are shown by dashed lines in order to control lightemission of the light-emitting element. Reference numeral 604 denotes asealing substrate, 605 denotes a sealant, and a portion surrounded bythe sealant 605 is a space 607.

A lead wiring 608 is a wiring for transmitting signals to be inputted tothe source side driver circuit 601 and the gate side driver circuit 603,and the lead wiring 608 receives video signals, clock signals, startsignals, reset signals, and the like from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is shown here, the FPC may be provided with a printed wiring board(PWB). The light-emitting device in this specification includes not onlya light-emitting device itself but also a light-emitting device with anFPC or a PWB attached thereto.

Next, a cross-sectional structure is explained with reference to FIG.5B. Although the driver circuit portion and the pixel portion are formedover an element substrate 610, FIG. 5B shows the source side drivercircuit 601 that is the driver circuit portion and one pixel in thepixel portion 602.

A CMOS circuit, which is a combination of an n-channel TFT 623 and ap-channel TFT 624, is formed as the source side driver circuit 601.Alternatively, the driver circuit may be formed using a CMOS circuit, aPMOS circuit, or an NMOS circuit. Although this embodiment mode shows adriver-integrated type where the driver circuit is formed over thesubstrate, the present invention is not limited to this, and the drivercircuit may be formed outside the substrate, not over the substrate.

The pixel portion 602 includes a plurality of pixels, each of whichincludes a switching TFT 611, a current controlling TFT 612, and a firstelectrode 613 electrically connected to a drain of the currentcontrolling TFT 612. Note that an insulator 614 is formed so as to coveran end portion of the first electrode 613. Here, a positivephotosensitive acrylic resin film is used for the insulator 614.

The insulator 614 is formed so as to have a curved surface havingcurvature at an upper end portion or a lower end portion thereof inorder to obtain favorable coverage. For example, in the case of using apositive photosensitive acrylic as a material for the insulator 614, theinsulator 614 is preferably formed so as to have a curved surface with acurvature radius (0.2 to 3 μm) only at the upper end portion thereof.Either a negative photosensitive acrylic which becomes insoluble in anetchant by light irradiation or a positive photosensitive acrylic whichbecomes soluble in an etchant by light irradiation can be used for theinsulator 614.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Various metals, alloys, electrically conductivecompounds, or a mixture thereof can be used for a material for formingthe first electrode 613. When the first electrode is used as an anode,it is preferable to use, among those materials, a metal, an alloy, anelectrically conductive compound, a mixture thereof, or the like with ahigh work function (a work function of greater than or equal to 4.0 eV).For example, a stacked-layer structure of a film containing titaniumnitride as its main component and a film containing aluminum as its maincomponent, a three-layer structure of a titanium nitride film, a filmcontaining aluminum as its main component, and a titanium nitride film,or the like can be used in addition to a single layer of indium tinoxide containing silicon, indium zinc oxide, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like. Whenthe first electrode 613 has a stacked layer structure, the resistance asa wiring is low and a favorable ohmic contact can be obtained.Furthermore, the first electrode 613 can function as an anode.

The EL layer 616 is formed by various methods such as an evaporationmethod using an evaporation mask, an ink-jet method, and a spin coatingmethod. The EL layer 616 has the light-emitting layer described inEmbodiment Mode 1. For other materials for forming the EL layer 616, alow molecular compound, oligomer, dendrimer, or a high molecularcompound may be used. In addition, not only an organic compound but alsoan inorganic compound may be used for the material for the EL layer 616.

Various metals, alloys, electrically conductive compounds, and mixturethereof can be used for a material for forming the second electrode 617.When the second electrode is used as a cathode, it is preferable to use,among those materials, a metal, an alloy, an electrically conductivecompound, a mixture thereof, or the like with a low work function (awork function of less than or equal to 3.8 eV). For example, an elementbelonging to Group 1 or Group 2 of the periodic table, that is, analkali metal such as lithium (Li) or cesium (Cs), an alkaline earthmetal such as magnesium (Mg), calcium (Ca), or strontium (Sr), an alloycontaining these (e.g., an MgAg alloy and an AlLi alloy), and the likecan be given. In the case where light generated in the EL layer 616 istransmitted through the second electrode 617, the second electrode 617may also be formed using stacked layers of a thin metal film and atransparent conductive film (e.g., indium tin oxide (ITO), indium tinoxide containing silicon or silicon oxide, indium zinc oxide (IZO), orindium oxide containing tungsten oxide and zinc oxide (IWZO)).

When the sealing substrate 604 and the element substrate 610 areattached to each other with the sealant 605, a light-emitting element618 is provided in the space 607 surrounded by the element substrate610, the sealing substrate 604, and the sealant 605. The space 607 isfilled with filler, and may be filled with an inert gas (e.g., nitrogenor argon), or the sealant 605.

An epoxy-based resin is preferably used for the sealant 605. Suchmaterial preferably allows as little moisture and oxygen as possible topenetrate. As a material for forming the sealing substrate 604, aplastic substrate made of FRP (fiberglass-reinforced plastics), PVF(polyvinyl fluoride), polyester, acrylic, or the like can be used aswell as a glass substrate or a quartz substrate.

In such a manner, the light-emitting device having the light-emittingelement of the present invention can be obtained.

The light-emitting device of the present invention has thelight-emitting element described in Embodiment Mode 1 or Embodiment Mode2. Thus, a light-emitting device with high light emission efficiency canbe obtained.

In addition, since the light-emitting element with high light emissionefficiency is included, a light-emitting device with low powerconsumption can be obtained.

Moreover, since a light-emitting element which emits light with goodcolor purity is included, a light-emitting device which is excellent incolor reproducibility can be obtained.

Furthermore, since a light-emitting element with less deterioration anda long lifetime is included, a light-emitting device with a longlifetime can be obtained.

Although an active matrix light-emitting device in which operation of alight-emitting element is controlled by a transistor is explained inthis embodiment mode as described above, a passive matrix light-emittingdevice may be alternatively used. FIG. 6A shows a perspective view of apassive matrix light-emitting device manufactured by application of thepresent invention. In FIGS. 6A and 6B, over a substrate 951, an EL layer955 is provided between an electrode 952 and an electrode 956. An edgeof the electrode 952 is covered with an insulating layer 953. Apartition layer 954 is provided over the insulating layer 953. Sidewalls of the partition layer 954 slope so that a distance between oneside wall and the other side wall becomes narrower toward a substratesurface. In other words, a cross section taken along the direction of ashorter side of the partition layer 954 has a trapezoidal shape, and abottom surface of the trapezoid (a side of the trapezoid which isparallel to the surface of the insulating layer 953 and is in contactwith the insulating layer 953) is shorter than a top surface (a side ofthe trapezoid which is parallel to the surface of the insulating layer953 and is not in contact with the insulating layer 953). The partitionlayer 954 provided in this manner can prevent the light-emitting elementfrom being defective due to static electricity or the like. In addition,the light-emitting element with high light emission efficiency of thepresent invention is included in a passive matrix light-emitting device,so that a light-emitting device with high light emission efficiency canbe obtained.

Note that this embodiment mode can be appropriately combined with otherembodiment modes.

Embodiment Mode 4

In this embodiment mode, an electronic appliance of the presentinvention, which includes the light-emitting device described inEmbodiment Mode 3 as part thereof will be explained. The electronicappliance of the present invention has the light-emitting elementdescribed in Embodiment Mode 1 or 2 and a display portion with a longlifetime.

As electronic appliances manufactured using the light-emitting device ofthe present invention, the following are given: cameras such as videocameras and digital cameras, goggle type displays, navigation systems,audio reproducing devices (e.g., car audio stereos and audio componentstereos), computers, game machines, portable information terminals(e.g., mobile computers, mobile phones, portable game machines, andelectronic books), and image reproducing devices provided with recordingmedia (specifically, a device capable of reproducing the content of arecording medium such as a digital versatile disc (DVD) and providedwith a display device that can display the reproduced image), and thelike. Specific examples of these electronic appliances are shown inFIGS. 7A to 7D.

FIG. 7A shows a television device of this embodiment mode, whichincludes a housing 9101, a support base 9102, a display portion 9103, aspeaker portion 9104, a video input terminal 9105, and the like. In thistelevision device, the display portion 9103 includes light-emittingelements similar to those described in Embodiment Modes 1 and 2, whichare arranged in matrix. The light-emitting element has features of highlight emission efficiency and low power consumption. In addition, thelight-emitting element has features of good color purity and a longlifetime. Since the display portion 9103 including the light-emittingelement also has the similar feature, this television device has lessdeterioration in image quality and low power consumption is achieved.With such features, the television device can have a significantlyreduced number or size of degradation correction functions and powersource circuits. Thus, reduction in size and weight of the housing 9101and the support base 9102 can be achieved. Since low power consumption,high image quality, and reduction in size and weight are achieved in thetelevision device of this embodiment mode, a product suitable for livingenvironment can be provided. In addition, the television device of thisembodiment mode has a display portion which is excellent in colorreproducibility; thus, clear images can be seen.

FIG. 7B shows a computer of this embodiment mode, which includes a mainbody 9201, a housing 9202, a display portion 9203, a keyboard 9204, anexternal connection port 9205, a pointing device 9206, and the like. Inthis computer, the display portion 9203 includes light-emitting elementssimilar to those described in Embodiment Modes 1 and 2, which arearranged in matrix. The light-emitting element has features of highlight emission efficiency and low power consumption. In addition, thelight-emitting element has features of excellent color purity and a longlifetime. Since the display portion 9203 including the light-emittingelement also has the similar feature, this computer has lessdeterioration in image quality and low power consumption is achieved.With such features, the computer can have a significantly reduced numberor size of degradation correction functions and power source circuits.Thus, reduction in size and weight of the main body 9201 and the housing9202 can be achieved. Since low power consumption, high image quality,and reduction in size and weight are achieved in the computer of thisembodiment mode, a product suitable for living environment can beprovided. In addition, the computer is portable, and a computer that hasa display portion which is able to withstand impacts by an externalsource that occur when it is being carried can be provided. Moreover,the computer of this embodiment mode has a display portion which isexcellent in color reproducibility; thus, clear images can be seen.

FIG. 7C shows a mobile phone of this embodiment mode, which includes amain body 9401, a housing 9402, a display portion 9403, an audio inputportion 9404, an audio output portion 9405, operation keys 9406, anexternal connection port 9407, an antenna 9408, and the like. In thismobile phone, the display portion 9403 includes light-emitting elementssimilar to those described in Embodiment Modes 1 and 2, which arearranged in matrix. The light-emitting element has features of highlight emission efficiency and low power consumption. In addition, thelight-emitting element has features of good color purity and a longlifetime. Since the display portion 9403 including the light-emittingelement also has the similar feature, this mobile phone has lessdeterioration in image quality and low power consumption is achieved.With such features, the mobile phone can have a significantly reducednumber or size of degradation correction functions and power sourcecircuits. Thus, reduction in size and weight of the main body 9401 andthe housing 9402 can be achieved. Since low power consumption, highimage quality, and reduction in size and weight are achieved in themobile phone of this embodiment mode, a product suitable for beingcarried can be provided. In addition, a product that has a display whichis able to withstand impacts by an external source that occur when it isbeing carried can be provided. Moreover, the mobile phone of thisembodiment mode has a display portion which is excellent in colorreproducibility; thus, clear images can be seen.

FIG. 7D shows a camera, which includes a main body 9501, a displayportion 9502, a housing 9503, an external connection port 9504, a remotecontrol receiving portion 9505, an image receiving portion 9506, abattery 9507, an audio input portion 9508, operation keys 9509, aneyepiece portion 9510, and the like. In this camera, the display portion9502 includes light-emitting elements similar to those described inEmbodiment Modes 1 and 2, which are arranged in matrix. Thelight-emitting element has features of high light emission efficiencyand low power consumption. In addition, the light-emitting element hasfeatures of good color purity and a long lifetime. Since the displayportion 9502 including the light-emitting element also has the similarfeature, this camera has less deterioration in image quality and lowpower consumption is achieved. With such features, the camera can have asignificantly reduced number or size of degradation correction functionsand power source circuits. Thus, reduction in size and weight of themain body 9501 can be achieved. Since low power consumption, high imagequality, and reduction in size and weight are achieved in the camera ofthis embodiment mode, a product suitable for being carried can beprovided. In addition, a product that has a display which is able towithstand impacts by an external source that occur when it is beingcarried can be provided. Moreover, the camera of this embodiment modehas a display portion which is excellent in color reproducibility; thus,clear images can be seen.

FIG. 8 shows an audio reproducing device, specifically, a car audiostereo, which includes a main body 701, a display portion 702, andoperation switches 703 and 704. The display portion 702 can be realizedby the (passive matrix or active matrix) light-emitting device describedin Embodiment Mode 3. Further, the display portion 702 may be formedusing a segment type light-emitting device. In any case, the use of thelight-emitting element of the present invention makes it possible toform a bright display portion with a long lifetime while achieving lowpower consumption with the use of vehicle power source (12 to 42 V). Inaddition, although this embodiment mode shows a car audio stereo, thelight-emitting element of the present invention can also be used for aportable or home audio device.

FIG. 9 shows a digital player as an example thereof. The digital playershown in FIG. 9 includes a main body 710, a display portion 711, amemory portion 712, an operation portion 713, earphones 714, and thelike. Headphones or wireless earphones can be used instead of theearphones 714. The display portion 711 can be realized by the (passivematrix or active matrix) light-emitting device described in EmbodimentMode 3. Further, the display portion 711 may be formed using a segmenttype light-emitting device. In any case, the use of the light-emittingelement of the present invention makes it possible to form a brightdisplay portion with a long lifetime which is capable of display evenwhen a secondary battery (a nickel hydride battery or the like) is used,while achieving low power consumption. The memory portion 712 is formedusing a hard disk or nonvolatile memory. For example, NAND typenonvolatile memory with recording capacity of 20 to 200 gigabytes (GBs)is used and the operation portion 713 is operated, so that an image orsound (e.g., music) can be recorded and reproduced. In the displayportions 702 and 711, white characters are displayed against a blackbackground, and thus, power consumption can be reduced. This iseffective especially in a mobile audio device.

As described above, the range of application of the light-emittingdevice manufactured by application of the present invention is verywide. Thus, the light-emitting device can be applied to electronicappliances in various fields. When the present invention is applied, anelectronic appliance with a highly reliable display portion with lowpower consumption can be provided.

In addition, the light-emitting device to which the present invention isapplied has a light-emitting element with high light emissionefficiency, and can be used as a lighting system. One mode of using thelight-emitting element to which the present invention is applied as alighting system is explained with reference to FIG. 10.

FIG. 10 shows an example of a liquid crystal display device using thelight-emitting device of the present invention as a backlight. Theliquid crystal display device shown in FIG. 10 includes a housing 901, aliquid crystal layer 902, a backlight 903, and a housing 904. The liquidcrystal layer 902 is connected to a driver IC 905. In addition, thelight-emitting device of the present invention is used for the backlight903, and current is supplied through a terminal 906.

When the light-emitting device of the present invention is used for abacklight of a liquid crystal display device, a backlight with highlight emission efficiency can be obtained. In addition, a backlight witha long lifetime can be obtained. The light-emitting device of thepresent invention is a lighting device with plane light emission, andcan have a large area. Thus, the backlight can have a large area and theliquid crystal display device can have a large area. Furthermore, sincethe light-emitting device of the present invention is thin and consumesless electric power, reduction in thickness and lower power consumptionof a display device can also be achieved.

FIG. 11 shows an example of using the light-emitting device of thepresent invention for a table lamp which is a lighting system. The tablelamp shown in FIG. 11 has a housing 2001 and a light source 2002, andthe light-emitting device of the present invention is used as the lightsource 2002. The light-emitting device of the present invention has along lifetime; thus, the table lamp also has a long lifetime.

FIG. 12 shows an example of using the light-emitting device of thepresent invention for an indoor lighting system 3001. Since thelight-emitting device of the present invention can have a large area,the light-emitting device of the present invention can be used as alighting system having a large emission area. In addition, since thelight-emitting device of the present invention has a long lifetime, thelight-emitting device of the present invention can be used as a lightingsystem with a long lifetime. When a television device 3002 of thepresent invention like the one shown in FIG. 7A is placed in a room inwhich the light-emitting device of the present invention is used as theindoor lighting system 3001, public broadcasting and movies can bewatched. In such a case, since both of the devices have long lifetimes,frequency of replacement of the lighting system and the televisiondevice can be reduced, and damages on the environment can be reduced.

Note that this embodiment mode can be appropriately combined with otherembodiment modes.

Embodiment 1

In this embodiment, a light-emitting element of the present inventionwill be specifically explained with reference to FIG. 13. Structuralformulas of organic compounds used in Embodiments 1 to 3 are shownbelow.

(Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2101 by sputtering, so that a first electrode2102 was formed. The thickness of the electrode 2102 was 110 nm and thearea was 2 mm×2 mm.

Next, the substrate over which the first electrode 2102 was formed wasfixed to a substrate holder which was provided in a vacuum evaporationapparatus so that the surface provided with the first electrode 2102faced down. After the pressure was reduced to about 10⁻⁴ Pa,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI) were co-evaporated over the first electrode 2102,so that a layer 2111 containing a composite material was formed. Thethickness of the layer 2111 was 50 nm. The weight ratio of NPB tomolybdenum oxide (VI) was adjusted to 4:1 (=NPB:molybdenum oxide). Notethat a co-evaporation method is a method in which evaporation from aplurality of evaporation sources is performed at the same time in onetreatment chamber.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was formed to have a thickness of 10 nm by an evaporation method usingresistance heating, so that a hole transporting layer 2112 was formed.

Next, a light-emitting layer 2113 was formed over the hole transportinglayer 2112. First, tris(8-quinolinolato)aluminum (abbreviation: Alq) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation Ir(Fdpq)₂(acac)) were co-evaporated over the holetransporting layer 2112, so that a first layer 2121 was formed with athickness of 10 nm. Here, the weight ratio of Alq to Ir(Fdpq)₂(acac) wasadjusted to 1:0.05 (=Alq:Ir(Fdpq)₂(acac)). In addition,2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)) were co-evaporated over the first layer2121, so that a second layer 2122 was formed with a thickness of 20 nm.Here, the weight ratio of BPAPQ to Ir(Fdpq)₂(acac) was adjusted to1:0.05 (=BPAPQ:Ir(Fdpq)₂(acac)).

After that, a film of tris(8-quinolinolato)aluminum (abbreviation: Alq)was formed to have a thickness of 10 nm over the light-emitting layer2113 by an evaporation method using resistance heating, so that anelectron transporting layer 2114 was formed.

Alq and lithium (Li) were co-evaporated over the electron transportinglayer 2114, so that an electron injecting layer 2115 with a thickness of50 nm was formed. Here, the weight ratio of Alq to lithium was adjustedto 1:0.01 (=Alq:lithium).

Finally, a film of aluminum was formed to have a thickness of 200 nm byan evaporation method using resistance heating, so that a secondelectrode 2104 was formed. In this manner, a light-emitting element 1was manufactured.

(Comparative Light-Emitting Element 2)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate by sputtering, so that a first electrode wasformed. The thickness of the first electrode was 110 nm and the area was2 mm×2 mm.

Next, the substrate over which the first electrode was formed was fixedto a substrate holder which was provided in a vacuum evaporationapparatus so that the surface provided with the first electrode faceddown. After the pressure was reduced to about 10⁻⁴ Pa,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI) were co-evaporated over the first electrode, sothat a layer containing a composite material was formed. The thicknessof the layer was 50 nm. The weight ratio of NPB to molybdenum oxide (VI)was adjusted to 4:1 (=NPB:molybdenum oxide). Note that a co-evaporationmethod is a method in which evaporation from a plurality of evaporationsources is performed at the same time in one treatment chamber.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was formed to have a thickness of 10 nm by an evaporation method usingresistance heating, so that a hole transporting layer was formed.

Next, a light-emitting layer was formed over the hole transportinglayer. 2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)) were co-evaporated, so that thelight-emitting layer was formed with a thickness of 30 nm. Here, theweight ratio of BPAPQ to Ir(Fdpq)₂(acac) was adjusted to 1:0.05(=BPAPQ:Ir(Fdpq)₂(acac)).

After that, a film of tris(8-quinolinolato)aluminum (abbreviation: Alq)was formed to have a thickness of 10 nm over the light-emitting layer byan evaporation method using resistance heating, so that an electrontransporting layer was formed.

Alq and lithium (Li) were co-evaporated over the electron transportinglayer, so that an electron injecting layer with a thickness of 50 nm wasformed. Here, the weight ratio of Alq to lithium was adjusted to 1:0.01(=Alq:lithium).

Finally, a film of aluminum was formed to have a thickness of 200 nm byan evaporation method using resistance heating, so that a secondelectrode was formed. In this manner, a comparative light-emittingelement 2 was manufactured.

FIG. 14 shows current density-luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.FIG. 15 shows voltage-luminance characteristics thereof. FIG. 16 showsluminance-current efficiency characteristics thereof. FIG. 17 showslight emission spectrums thereof when current of 1 mA was fed.

The CIE chromaticity coordinates of the comparative light-emittingelement 2 at a luminance of 760 cd/m² were (x, y)=(0.64, 0.34), andemission of red light was obtained. The current efficiency, voltage,current density, power efficiency, and external quantum efficiency atthe luminance of 760 cd/m² were 3.2 cd/A, 4.8 V, 23.6 mA/cm², 5.6(lm/W), and 5.6%, respectively.

On the other hand, the CIE chromaticity coordinates of thelight-emitting element 1 at a luminance of 1070 cd/m² were (x, y)=(0.70,0.30), and emission of deep red light was obtained. The currentefficiency, voltage, current density, power efficiency, and externalquantum efficiency at the luminance of 1070 cd/m² were 4.8 cd/A, 7.4 V,22.6 mA/cm², 2.0 (lm/W), and 10%, respectively.

Accordingly, the current efficiency and the external quantum efficiencyof the light-emitting element 1 to which the present invention isapplied are higher than those of the comparative light-emitting element2. In addition, the light-emitting element 1 is a light-emitting elementin which the power efficiency is higher and power consumption is lessthan those of the comparative light-emitting element 2.

Moreover, according to FIG. 17, the peak of the light emission spectrumin the comparative light-emitting element 2, which was observed at near500 to 550 nm, was not observed in the light-emitting element 1. Thatis, in the light-emitting element 1 to which the present invention wasapplied, light emission from Alq that was the electron transportinglayer could be suppressed and light emission with good color puritycould be obtained.

Accordingly, application of the present invention makes it possible toobtain a light-emitting element with improved carrier balance and highlight emission efficiency. In addition, a light-emitting element whichconsumes less electric power could be obtained. Moreover, alight-emitting element which emits light with good color purity could beobtained.

Embodiment 2

In this embodiment, a light-emitting element of the present inventionwill be specifically explained with reference to FIG. 13.

(Light-Emitting Element 3)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2101 by sputtering, so that a first electrode2102 was formed. The thickness of the first electrode 2102 was 110 nmand the area was 2 mm×2 mm.

Next, the substrate over which the first electrode 2102 was formed wasfixed to a substrate holder which was provided in a vacuum evaporationapparatus so that the surface provided with the first electrode 2102faced down. After the pressure was reduced to about 10⁻⁴ Pa,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI) were co-evaporated over the first electrode 2102,so that a layer 2111 containing a composite material was formed. Thethickness of the layer 2111 was 50 nm. The weight ratio of NPB tomolybdenum oxide (VI) was adjusted to 4:1 (=NPB:molybdenum oxide). Notethat a co-evaporation method is a method in which evaporation from aplurality of evaporation sources is performed at the same time in onetreatment chamber.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was formed to have a thickness of 10 nm by an evaporation method usingresistance heating, so that a hole transporting layer 2112 was formed.

Next, a light-emitting layer 2113 was formed over the hole transportinglayer 2112. First, tris(8-quinolinolato)aluminum (abbreviation: Alq) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)) were co-evaporated over the holetransporting layer 2112, so that a first layer 2121 was formed with athickness of 15 nm. Here, the weight ratio of Alq to Ir(Fdpq)₂(acac) wasadjusted to 1:0.1 (=Alq:Ir(Fdpq)₂(acac)). In addition,2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(II)(abbreviation: Ir(Fdpq)₂(acac)) were co-evaporated over the first layer2121, so that a second layer 2122 was formed with a thickness of 15 nm.Here, the weight ratio of BPAPQ to Ir(Fdpq)₂(acac) was adjusted to 1:0.1(=BPAPQ:Ir(Fdpq)₂(acac)).

After that, by an evaporation method using resistance heating, a film oftris(8-quinolinolato)aluminum (abbreviation: Alq) was formed to have athickness of 15 nm over the light-emitting layer 2113, and a film ofbathophenanthroline (abbreviation: BPhen) was formed to have a thicknessof 30 nm, so that an electron transporting layer 2114 in which twolayers were stacked was formed.

After that, a film of lithium fluoride (LiF) was formed to have athickness of 1 nm over the electron transporting layer 2114 by anevaporation method using resistance heating, so that an electroninjecting layer 2115 was formed.

Finally, a film of aluminum was formed to have a thickness of 200 nm byan evaporation method using resistance heating, so that a secondelectrode 2104 was formed. In this manner, a light-emitting element 3was manufactured.

FIG. 18 shows current density-luminance characteristics of thelight-emitting element 3. FIG. 19 shows voltage-luminancecharacteristics thereof. FIG. 20 shows luminance-current efficiencycharacteristics thereof. FIG. 21 shows light emission spectrums thereofwhen current of 1 mA was fed.

The CIE chromaticity coordinates of the light-emitting element 3 at aluminance of 1030 cd/m² were (x, y)=(0.70, 0.30), and emission of deepred light was obtained. The current efficiency, voltage, and currentdensity at the luminance of 1030 cd/m² were 5.2 cd/A, 6.1 V, and 20.0mA/cm², respectively.

FIG. 22 shows the result of a continuous lighting test in which thelight-emitting element 3 was continuously lit by constant currentdriving with an initial luminance set at 1000 cd/m². A vertical axisindicates the relative luminance (normalized luminance) on theassumption that 1000 cd/m² is 100%. When a continuous lighting test wasconducted in which the light-emitting element 3 was continuously lit byconstant current driving with the initial luminance set at 1000 cd/m²,91% of the initial luminance was obtained after 630 hours. In addition,when time when luminance reached 90% of the initial luminance wasroughly estimated, it was about 700 hours, and when time when luminancereached 50% of the initial luminance (half-life) was roughly estimated,it was about 20000 hours. Accordingly, the light-emitting element 3 wasa light-emitting element with a very long lifetime.

Accordingly, application of the present invention makes it possible toobtain a light-emitting element with improved carrier balance and highlight emission efficiency. In addition, a light-emitting element whichconsumes less electric power could be obtained. Moreover, alight-emitting element with a long lifetime could be obtained.

Embodiment 3

In this embodiment, a light-emitting element of the present inventionwill be specifically explained with reference to FIG. 13.

(Light-Emitting Element 4)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2101 by sputtering, so that a first electrode2102 was formed. The thickness of the first electrode 2102 was 110 nmand the area was 2 mm×2 mm.

Next, the substrate over which the first electrode 2102 was formed wasfixed to a substrate holder which was provided in a vacuum evaporationapparatus so that the surface provided with the first electrode 2102faced down. After the pressure was reduced to about 10⁻⁴ Pa,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI) were co-evaporated over the first electrode 2102,so that a layer 2111 containing a composite material was formed. Thethickness of the layer 2111 was 50 nm. The weight ratio of NPB tomolybdenum oxide (VI) was adjusted to 4:1 (=NPB:molybdenum oxide). Notethat a co-evaporation method is a method in which evaporation from aplurality of evaporation sources is performed at the same time in onetreatment chamber.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was formed to have a thickness of 10 nm by an evaporation method usingresistance heating, so that a hole transporting layer 2112 was formed.

Next, a light-emitting layer 2113 was formed over the hole transportinglayer 2112. First,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq) and (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)) were co-evaporated over the holetransporting layer 2112, so that a first layer 2121 was formed with athickness of 10 nm. Here, the weight ratio of BAlq to Ir(tppr)₂(acac)was adjusted to 1:0.06 (=BAlq:Ir(tppr)₂(acac)). In addition,2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)) were co-evaporated over the first layer2121, so that a second layer 2122 was formed with a thickness of 20 nm.Here, the weight ratio of BPAPQ to Ir(tppr)₂(acac) was adjusted to1:0.06 (=BPAPQ:Ir(tppr)₂(acac)).

After that, a film of tris(8-quinolinolato)aluminum (abbreviation: Alq)was formed to have a thickness of 10 nm over the light-emitting layer2113 by an evaporation method using resistance heating, so that anelectron transporting layer 2114 was formed.

Alq and lithium (Li) were co-evaporated over the electron transportinglayer 2114, so that an electron injecting layer 2115 with a thickness of50 nm was formed. Here, the weight ratio of Alq to lithium was adjustedto 1:0.01 (=Alq:lithium).

Finally, a film of aluminum was formed to have a thickness of 200 nm byan evaporation method using resistance heating, so that a secondelectrode 2104 was formed. In this manner, a light-emitting element 4was manufactured.

(Comparative Light-Emitting Element 5)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate by sputtering, so that a first electrode wasformed. The thickness of the first electrode was 110 nm and the area was2 mm×2 mm.

Next, the substrate over which the first electrode was formed was fixedto a substrate holder which was provided in a vacuum evaporationapparatus so that the surface provided with the first electrode faceddown. After the pressure was reduced to about 10⁻⁴ Pa,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI) were co-evaporated over the first electrode, sothat a layer containing a composite material was formed. The thicknessof the layer was 50 nm. The weight ratio of NPB to molybdenum oxide (VI)was adjusted to 4:1 (=NPB:molybdenum oxide). Note that a co-evaporationmethod is a method in which evaporation from a plurality of evaporationsources is performed at the same time in one treatment chamber.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was formed to have a thickness of 10 nm by an evaporation method usingresistance heating, so that a hole transporting layer was formed.

Next, a light-emitting layer was formed over the hole transportinglayer. 2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)) were co-evaporated, so that thelight-emitting layer was formed with a thickness of 30 nm. Here, theweight ratio of BPAPQ to Ir(tppr)₂(acac) was adjusted to 1:0.06 (=BPAPQ:Ir(tppr)₂(acac)).

After that, a film of tris(8-quinolinolato)aluminum (abbreviation: Alq)was formed to have a thickness of 10 nm over the light-emitting layer byan evaporation method using resistance heating, so that an electrontransporting layer was formed.

Alq and lithium (Li) were co-evaporated over the electron transportinglayer, so that an electron injecting layer with a thickness of 50 nm wasformed. Here, the weight ratio of Alq to lithium was adjusted to 1:0.01(=Alq:lithium).

Finally, a film of aluminum was formed to have a thickness of 200 nm byan evaporation method using resistance heating, so that a secondelectrode was formed. In this manner, a comparative light-emittingelement 5 was manufactured.

FIG. 23 shows current density-luminance characteristics of thelight-emitting element 4 and the comparative light-emitting element 5.FIG. 24 shows voltage-luminance characteristics thereof. FIG. 25 showsluminance-current efficiency characteristics thereof. FIG. 26 showslight emission spectrums thereof when current of 1 mA was fed.

The CIE chromaticity coordinates of the comparative light-emittingelement 5 at a luminance of 1120 cd/m² were (x, y)=(0.63, 0.36), andemission of orange-red light was obtained. The current efficiency,voltage, current density, power efficiency, and external quantumefficiency at the luminance of 1120 cd/m² were 6.7 cd/A, 4.0 V, 16.8mA/cm², 5.2 (lm/W), and 4.7%, respectively.

On the other hand, the CIE chromaticity coordinates of thelight-emitting element 4 at a luminance of 980 cd/m² were (x, y)=(0.65,0.35), and emission of red light was obtained. The current efficiency,voltage, current density, power efficiency, and external quantumefficiency at the luminance of 980 cd/m² were 9.5 cd/A, 5.2 V, 10.3mA/cm², 5.8 (lm/W), and 7.0%, respectively.

Accordingly, the current efficiency and the external quantum efficiencyof the light-emitting element 4 to which the present invention isapplied are higher than those of the comparative light-emitting element5. In addition, the light-emitting element 4 is a light-emitting elementin which the power efficiency is higher and power consumption is lessthan those of the comparative light-emitting element 5.

Moreover, according to FIG. 26, the peak of the light emission spectrumin the comparative light-emitting element 5, which was observed at near500 to 550 nm, was not observed in the light-emitting element 4. Thatis, in the light-emitting element 4 to which the present invention wasapplied, light emission from Alq that was the electron transportinglayer could be suppressed and light emission with good color puritycould be obtained.

Accordingly, application of the present invention makes it possible toobtain a light-emitting element with improved carrier balance and highlight emission efficiency. In addition, a light-emitting element whichconsumes less electric power could be obtained. Moreover, alight-emitting element which emits light with good color purity could beobtained.

Embodiment 4

In this embodiment, the reduction reaction characteristics of2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)), and(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)) which were used for the light-emittinglayers of the light-emitting element 1, light-emitting element 3, andlight-emitting element 4 to which the present invention was applied weremeasured by cyclic voltammetry (CV) measurement. In addition, the LUMOlevels of BPAPQ, Ir(Fdpq)₂(acac), and Ir(tppr)₂(acac) were obtained fromthe measurement results. Note that an electrochemical analyzer (ALSmodel 600A, manufactured by BAS Inc.) was used for the measurement.

A solution for the CV measurement was prepared in such a manner thatdehydrated dimethylformamide (DMF) (manufactured by Sigma-Aldrich Corp.,99.8%, catalogue number: 22705-6,) was used for a solvent,tetra-n-buthylammonium perchlorate (n-Bu₄NClO₄) (manufactured by TokyoChemical Industry Co., Ltd., catalog number: T0836), which wassupporting electrolyte, was dissolved so as to be contained at aconcentration of 100 mmol/L, and furthermore, an object to be measuredwas dissolved so as to be contained at a concentration of 1 mmol/L. Aplatinum electrode (manufactured by BAS Inc., PTE platinum electrode)was used as a working electrode, a platinum electrode (manufactured byBAS Inc., Pt counter electrode for VC-3, (5 cm)) was used as anauxiliary electrode, and an Ag/Ag⁺ electrode (manufactured by BAS Inc.,RE-5 reference electrode for nonaqueous solvent) was used as a referenceelectrode. Note that the measurement was conducted at room temperature(20 to 25° C.).

(Calculation of the Potential Energy of the Reference Electrode withRespect to the Vacuum Level)

First, potential energy (eV) of the reference electrode (Ag/Ag⁺electrode) used in this embodiment with respect to the vacuum level wascalculated. That is, the Fermi level of the Ag/Ag⁺ electrode wascalculated. It is known that the oxidation-reduction potential offerrocene in methanol is +0.610 [V vs. SHE] (Reference: Christian R.Goldsmith et al., J. Am. Chem. Soc., Vol. 124, No. 1, pp. 83-96, 2002).On the other hand, when the oxidation-reduction potential of ferrocenein methanol was calculated using the reference electrode used in thisembodiment, the result was +0.20 V [vs. Ag/Ag+]. Thus, it was found thatthe potential energy of the reference electrode used in this embodimentwas lower than that of the standard hydrogen electrode by 0.41 [eV].

Here, it is also known that the potential energy of the standardhydrogen electrode with respect to the vacuum level is −4.44 eV(Reference: Toshihiro Ohnishi and Tamami Koyama, High Molecular ELMaterial (Kyoritsu shuppan), pp. 64-67). Accordingly, the potentialenergy of the reference electrode used in this embodiment with respectto the vacuum level could be calculated to be −4.44−0.41=−4.85 [eV].

Measurement Example 1 BPAPQ

In Measurement Example 1, the reduction reaction characteristics ofBPAPQ were observed by cyclic voltammetry (CV) measurement. 100 cyclesof measurement was conducted at a scan speed of 0.1 V/sec.

FIG. 28 shows the measurement result of the reduction reactioncharacteristics. Note that in the measurement of the reduction reactioncharacteristics, the scan in which the electric potential of a workingelectrode with respect to a reference electrode was changed from −0.56to −2.20 V, and then changed from −2.20 to −0.56 V was assumed as onecycle.

As shown in FIG. 28, a reduction peak potential E_(pc) was −1.95 V andan oxidation peak potential E_(pa) was −1.86 V. Accordingly, a half-wavepotential (an intermediate potential between E_(pc) and E_(pa)) can becalculated to be −1.91 V. This shows that BPAPQ can be reduced byelectrical energy of −1.91 [V vs. Ag/Ag⁺], and this energy correspondsto the LUMO level. Here, the potential energy of the reference electrodeused in this embodiment with respect to the vacuum level is −4.85 [eV]as described above. Thus, it was found that the LUMO level of BPAPQ was−4.85−(−1.91)=−2.94 [eV].

Measurement Example 2 Ir(Fdpq)₂(acac)

In Measurement Example 2, the reduction reaction characteristics ofIr(Fdpq)₂(acac) were observed by cyclic voltammetry (CV) measurement.100 cycles of measurement was conducted at a scan speed of 0.1 V/sec.

FIG. 29 shows the measurement result of the reduction reactioncharacteristics. Note that in the measurement of the reduction reactioncharacteristics, the scan in which the electric potential of the workingelectrode with respect to the reference electrode was changed from −0.40to −2.40 V, and then changed from −2.40 to −0.40 V was assumed as onecycle.

As shown in FIG. 29, a reduction peak potential E_(pc) was −1.58 V andan oxidation peak potential E_(pa) was −1.51 V. Accordingly, a half-wavepotential (an intermediate potential between E_(pc) and E_(pa)) can becalculated to be −1.55 V. This shows that Ir(Fdpq)₂(acac) can be reducedby electrical energy of −1.55 [V vs. Ag/Ag⁺], and this energycorresponds to the LUMO level. Here, the potential energy of thereference electrode used in this embodiment with respect to the vacuumlevel is −4.85 [eV] as described above. Thus, it was found that the LUMOlevel of Ir(Fdpq)₂(acac) was −4.85−(−1.55)=−3.30 [eV].

Measurement Example 3 Ir(tppr)₂(acac)

In Measurement Example 3, the reduction reaction characteristics ofIr(Fdpq)₂(acac) were observed by cyclic voltammetry (CV) measurement.100 cycles of measurement was conducted at a scan speed of 0.1 V/sec.

FIG. 30 shows the measurement result of the reduction reactioncharacteristics. Note that in the measurement of the reduction reactioncharacteristics, the scan in which the electric potential of the workingelectrode with respect to the reference electrode was changed from −0.34to −2.40 V, and then changed from −2.40 to −0.34 V was assumed as onecycle.

As shown in FIG. 30, a reduction peak potential E_(pc) was −1.88 V andan oxidation peak potential E_(pa) was −1.82 V. Accordingly, a half-wavepotential (an intermediate potential between E_(pc) and E_(pa)) can becalculated to be −1.85 V. This shows that Ir(tppr)₂(acac) can be reducedby electrical energy of −1.85 [V vs. Ag/Ag⁺], and this energycorresponds to the LUMO level. Here, the potential energy of thereference electrode used in this embodiment with respect to the vacuumlevel is −4.85 [eV] as described above. Thus, it was found that the LUMOlevel of Ir(tppr)₂(acac) was −4.85−(−1.85)=−3.00 [eV].

Consequently, it is found that a value of a difference between the LUMOlevels of BPAPQ and Ir(Fdpq)₂(acac) is −2.94−(−3.30)=0.36 [eV].Accordingly, Ir(Fdpq)₂(acac) functions as an electron trap in thelight-emitting element 1 manufactured in Embodiment 1 and thelight-emitting element 3 manufactured in Embodiment 2. Thus,Ir(Fdpq)₂(acac) can be preferably used in the present invention. Inparticular, the value of the difference in the LUMO levels is greaterthan or equal to 0.3 eV, so that a greater effect can be obtained.Specifically, effects that carrier balance is further improved and lightemission efficiency is high can be obtained.

Moreover, it is found that a value of a difference between the LUMOlevels of BPAPQ and Ir(tppr)₂(acac) is −2.94−(−3.00)=0.06 [eV].Accordingly, it is found that Ir(tppr)₂(acac) functions as an electrontrap in the light-emitting element 4 manufactured in Embodiment 3. Thus,Ir(tppr)₂(acac) can be preferably used in the present invention.

This application is based on Japanese Patent Application serial no.2006-322425 filed in Japan Patent Office on Nov. 29, 2006, the entirecontents of which are hereby incorporated by reference.

1. A light-emitting element comprising: an anode; a first layercontaining a first organic compound and a third organic compound overthe anode; a second layer containing a second organic compound and thethird organic compound over the first layer; and a cathode over thesecond layer, wherein the first layer is in contact with the secondlayer on the anode side, wherein the first and second layers are alight-emitting layer, wherein the first organic compound includes anelectron transporting property, wherein the second organic compoundincludes a hole transporting property, and wherein the third organiccompound includes an electron trapping property.
 2. A light-emittingelement comprising: an anode; a first layer containing a first organiccompound and a third organic compound over the anode; a second layercontaining a second organic compound and the third organic compound overthe first layer; and a cathode over the second layer, wherein the firstlayer is in contact with the second layer on the anode side, wherein thefirst and second layers are a light-emitting layer, wherein the firstorganic compound includes an electron transporting property, wherein thesecond organic compound includes a hole transporting property, andwherein a lowest unoccupied molecular orbital level of the third organiccompound is lower than that of the second organic compound by greaterthan or equal to 0.3 eV.
 3. The light-emitting element according toclaim 1, wherein an electron transporting layer and a hole transportinglayer are interposed between the anode and the cathode.
 4. Thelight-emitting element according to claim 2, wherein an electrontransporting layer and a hole transporting layer are interposed betweenthe anode and the cathode.
 5. The light-emitting element according toclaim 1, wherein the first organic compound is a metal complex.
 6. Thelight-emitting element according to claim 2, wherein the first organiccompound is a metal complex.
 7. The light-emitting element according toclaim 1, the second organic compound is an organic compound with abipolar property.
 8. The light-emitting element according to claim 2,the second organic compound is an organic compound with a bipolarproperty.
 9. The light-emitting element according to claim 1, whereinthe second organic compound is an arylamine derivative or a carbazolederivative.
 10. The light-emitting element according to claim 2, whereinthe second organic compound is an arylamine derivative or a carbazolederivative.
 11. The light-emitting element according to claim 1, whereinthe third organic compound is a substance emitting phosphorescence. 12.The light-emitting element according to claim 2, wherein the thirdorganic compound is a substance emitting phosphorescence.
 13. Thelight-emitting element according to claim 1, wherein the third organiccompound is a metal complex having a pyrazine skeleton or a quinoxalineskeleton, and has a metal atom of Group 9 or Group 10 of the periodictable.
 14. The light-emitting element according to claim 2, wherein thethird organic compound is a metal complex having a pyrazine skeleton ora quinoxaline skeleton, and has a metal atom of Group 9 or Group 10 ofthe periodic table.
 15. A method for manufacturing a light-emittingelement, comprising the steps of: forming an anode; forming a firstlayer containing a first organic compound and a third organic compoundover the anode; forming a second layer containing a second organiccompound and the third organic compound over the first layer; andforming a cathode over the second layer, wherein the first layer is incontact with the second layer on the first electrode side, wherein thefirst and second layers are a light-emitting layer, wherein the firstorganic compound is an organic compound with an electron transportingproperty, wherein the second organic compound is an organic compoundwith a hole transporting property, and wherein the third organiccompound includes an electron trapping property.
 16. The method formanufacturing a light-emitting element according to claim 15, wherein anelectron transporting layer and a hole transporting layer are interposedbetween the anode and the cathode.
 17. The method for manufacturing alight-emitting element according to claim 15, wherein the second organiccompound is an organic compound with a bipolar property.
 18. The methodfor manufacturing a light-emitting element according to claim 15,wherein a lowest unoccupied molecular orbital level of the third organiccompound is lower than that of the second organic compound by greaterthan or equal to 0.3 eV.
 19. The method for manufacturing alight-emitting element according to claim 15, wherein the third organiccompound is a substance emitting phosphorescence.
 20. A light-emittingdevice comprising: a first substrate; a thin film transistor over thefirst substrate; an insulating film over the thin film transistor; afirst electrode electrically connected to the thin film transistor overthe insulating film; a light-emitting layer having a first layer and asecond layer over the first electrode; a second electrode over thelight-emitting layer; a sealing material over the first substrate; and asecond substrate opposite to the first substrate, wherein the firstlayer contains a first organic compound and a third organic compound,wherein the second layer contains a second organic compound and thethird organic compound, wherein the first layer is in contact with thesecond layer on the first electrode side, wherein the first organiccompound is an organic compound with an electron transporting property,wherein the second organic compound is an organic compound with a holetransporting property, and wherein the third organic compound has anelectron trapping property.
 21. The method for manufacturing alight-emitting element according to claim 20, wherein an electrontransporting layer and a hole transporting layer are interposed betweenthe first electrode and the second electrode.
 22. The light-emittingdevice according to claim 20, wherein the second organic compound is anorganic compound with a bipolar property.
 23. The light-emitting deviceaccording to claim 20, wherein a lowest unoccupied molecular orbitallevel of the third organic compound is lower than that of the secondorganic compound by greater than or equal to 0.3 eV.
 24. Thelight-emitting device according to claim 20, wherein the third organiccompound is a substance emitting phosphorescence.